Gas detectors which operate and monitor gas concentrations in a fixed location are known in the industry as point detectors. Point gas detectors have traditionally employed catalytic beads, electrochemical cells and other chemical sensors such as MOS sensors depending on gas type and concentration. These types of sensors suffer from various drawbacks such as poisoning by other chemicals, the need for oxygen to function, limited operating temperatures, drift with time and the requirement for periodic field calibration. Another major drawback is that the sensors are not fail-to-safe, i.e. they do not inform the user when they are dysfunctional.
In recent years, optical based gas sensors have been introduced in the marketplace with the bulk of products utilizing infrared wavelengths to measure gases such as hydrocarbons, carbon dioxide and carbon monoxide. The infrared region is a preferred region since these gases have strong absorption due to fundamental vibrational-rotational bands as at 3.3 microns for methane and 4.2 microns for carbon dioxide. In optical gas sensors signals are present at all concentrations of gas including zero gas. As a result, it is easy to detect a fault in the system leading to the fail-to-safe operation. In contrast, catalytic sensors tend to simply lose their sensitivity when poisoned without the knowledge of the user.
Toxic gases such as hydrogen sulfide and sulfur dioxide occur in nature accompanying natural gas, and are also used extensively in the chemical industry. Unlike hydrocarbons which are combustible and need to be measured in LEL's (lower explosive limit) of a few percent by volume, toxic gases are unsafe for humans to breathe in much smaller doses typically of parts per million (ppm). Hence, these gases need to be measured in parts per million, and sometimes as with benzene in sub-ppm or parts per billion (ppb) concentrations. Detailed laboratory studies have shown that most of these toxic gases have very weak absorption in the infrared which, combined with the requirement of parts per million detection, make gas detectors operating in the infrared unsuited to these gases.
Studies in another part of the electromagnetic spectrum, namely the ultraviolet region, have shown that many toxic gases are strongly absorbing in the ultraviolet (uv). The region from 190 to 230 nanometers (nm) is strongly absorbing for hydrogen sulfide, sulfur dioxide, and the aromatics benzene, toluene and xylene. Below 190 nm the atmosphere strongly absorbs the deep ultraviolet radiation; the region below 190 nm is therefore known as the vacuum ultraviolet (vuv) region. Though ultraviolet based instruments are used in the process industry in flow measurements of toxic gases, no human and animal point safety product based on ultraviolet absorption principles is known to exist in the marketplace. U.S. Pat. No. 3,795,812 issued Mar. 5, 1974 and U.S. Pat. No. 3,906,226 issued Sept. 16, 1975 describe ultraviolet fluorescence monitors used to measure and control the emission of sulfur dioxide and nitric oxide respectively. Fluorescence monitoring is based on the efficient re-emission of radiation when these compounds are excited by light, and is a different technique from a non-dispersive absorption technique in accordance with this invention.
It would therefore represent an advance in the art to provide an optical absorption instrument utilizing absorption in the 190 to 230 nm region as an active indicator of gas, and using a second wavelength centered at 280 nm where the toxic gases are not absorbing as a reference channel.